Exemplary embodiments of the present invention are directed to a method for determining a quality of a reducing agent solution containing ammonia used for reducing nitrogen oxides. Specifically, exemplary embodiments of the present invention relate to a method for determining a quality of a reducing agent solution containing ammonia used for reducing nitrogen oxides in an SCR exhaust emission control system of an internal combustion engine.
For the catalytically assisted removal of nitrogen oxides (NOx) from internal combustion engine exhaust gas, it is known to add aqueous urea solution as a reducing agent containing ammonia (NH3) to the exhaust gas of the internal combustion engine. Due to thermolysis and/or hydrolysis, NH3 is released into the hot exhaust gas as the actual selective reducing agent with regard to the NOx reduction in a so-called SCR catalytic converter. This results in the problem of determining an appropriate metering quantity of the reducing agent for the intended cleaning effect, while also avoiding overmetering, which may result in undesirable NH3 slip. Furthermore, by its nature the concentration of ammonia or urea in the reducing agent significantly influences the metering quantity to be set. To take this influencing variable into account, various sensor-based diagnostic methods have already been proposed for ascertaining the quality of the reducing agent solution determined by the ammonia or urea concentration, but they involve correspondingly complex hardware.
To avoid this complexity, German patent document DE 102007022594 proposes to evaluate the quality of the reducing agent solution under certain operating conditions based on ammonia slip determined downstream from the SCR catalytic converter. In general, however, it is desirable to keep the emissions of pollutants in the exhaust gas of internal combustion engines, and thus also ammonia slip, as low as possible.
Exemplary embodiments of the present invention, therefore, provide a method by means of which the most reliable assessment possible may be made of an in particular decreased quality of a reducing agent solution used for reducing nitrogen oxides, in a manner which is less complex with regard to equipment and which largely avoids harmful emissions.
In the method according to the invention for determining a quality of a reducing agent solution containing ammonia used for reducing nitrogen oxides in an SCR exhaust emission control system of an internal combustion engine, a metering unit for delivering predefinable target metering quantities of the reducing agent solution into the internal combustion engine exhaust gas is actuated, and by means of a NOx sensor situated downstream from an SCR catalytic converter of the SCR exhaust emission control system and which has a cross-sensitivity to ammonia, an efficiency value that is at least correlated with the efficiency of the SCR catalytic converter is determined and compared to a predefinable limit value. A first target metering quantity and a first limit value in a normal metering mode are provided in a first method step. In the event that an efficiency value not corresponding to proper functioning of the SCR exhaust emission control system is identified by the comparison, the normal metering mode is terminated and a switch is made to a diagnostic mode in a second method step, in which for a first time period, a second target metering quantity that is greater than the first metering quantity, and a second limit value are provided. If a predefinable deviation of the efficiency value from the second limit value is identified after the first time period has elapsed, conditioning of the SCR catalytic converter is carried out in a third method step in such a way that a quantity of ammonia stored in the SCR catalytic converter falls below a predefinable storage quantity limit value. Subsequently, in an adaptation mode as the fourth method step, a third target metering quantity and a third limit value are provided for a predefinable second time period, and if a predefinable deviation of the efficiency value from the third limit value is identified after the second time period has elapsed, a deficient quality of the reducing agent solution is diagnosed.
Urea, ammonium carbamate, ammonium formate, and/or a similar substance that is capable of splitting NH3 and that is present in the form of an aqueous solution is primarily suitable as the reducing agent. The reducing agent solution may be added to the exhaust gas in pure form or as an aerosol-like mist by means of a metering unit situated upstream from the SCR catalytic converter in an appropriate exhaust gas line of the SCR exhaust emission control system. The metering unit is preferably designed in the manner of a metering valve having one or more nozzle openings. The SCR catalytic converter is preferably designed as a honeycomb structure in the form of an extruded bulk catalyst based on tungsten oxide or vanadium pentoxide, or as a coated zeolitic catalyst based on a copper- or iron-containing zeolite. It is known that these types of catalytic converters are able to store considerable quantities of NH3, whereby stored NH3 acts as a reactant for reduction of NOx contained in the exhaust gas. The NH3 quantity stored in the SCR catalytic converter is also referred to below as an NH3 filling level. The NH3 filling level is preferably considered to be a relative quantity that indicates the stored NH3 quantity with respect to the maximum storable NH3 quantity under the instantaneous conditions.
An efficiency value that is at least correlated with the efficiency of the SCR catalytic converter is determined by means of the NOx sensor, which is situated downstream from the SCR catalytic converter and which is sensitive to NOx and NH3. As the result of a comparison, which is generally and preferably continuous or repetitive and is carried out during brief time intervals, of the determined efficiency value to a predefinable limit value, in particular a predefinable first limit value in the normal metering mode, monitoring of the functioning of the SCR exhaust emission control system and in particular of the SCR catalytic converter is made possible. The efficiency value is preferably an efficiency regarding the NOx conversion capability of the SCR catalytic converter, or is represented by the signal value of the NOx sensor itself. In addition, by means of the NOx sensor, the NH3 filling level of the SCR catalytic converter, which is crucial for the NOx conversion, may be monitored and used as the efficiency value. In the normal metering mode, the metering quantity of the reducing agent is continuously adjusted in such a way that the efficiency value is within a predefinable value range. An insufficient NOx conversion or undermetering of the reducing agent may be recognized by an increased NOx concentration, for example, and corrected if necessary by adjusting the metering quantity. A decreased NH3 storage capacity of the SCR catalytic converter or overmetering may be recognized by an increased NH3 concentration, and if necessary may likewise be corrected by adjusting the metering quantity.
If the determined efficiency value is within a predefinable range about a target value, proper functioning of the SCR exhaust emission control system may be assumed, and there is generally no need for interrupting or terminating the normal metering mode. However, if the comparison shows, in particular due to an exceedance of the first limit value, that the SCR exhaust emission control system is not functioning properly, this is treated as a malfunction, which according to the invention is further analyzed. In the case of a concentration detected by the NOx sensor as the efficiency value, the first limit value is preferably in a range between 5 ppm and 50 ppm of NOx or NH3.
According to the invention, in the event that improper functioning of the SCR exhaust emission control system is identified, the normal metering mode is terminated and a switch is made to a diagnostic mode in which for a first time period, a second target metering quantity which is greater than the first metering quantity, and a second limit value are provided. The second limit value is preferably specified as a function of the increase in the target metering quantity. The signal pattern of the NOx sensor as a response to the increased target metering quantity is thus evaluated, and based on the evaluation a decision is made no later than the elapsing of the first time period as to whether the malfunction initially detected requires further diagnosis. According to the invention, if a predefinable deviation of the determined efficiency value from the second limit value is identified after the first time period has elapsed, a method step for conditioning the SCR catalytic converter is initiated and carried out. In this conditioning step, as the result of suitable metering of the reducing agent and/or a suitable NOx supply, the NH3 quantity stored in the SCR catalytic converter falls below a predefinable storage quantity limit value. The storage quantity limit value is preferably selected such that slip of the reducing agent or NH3 may be ruled out with sufficient certainty. An increased quantity of NH3 stored in the SCR catalytic converter possibly caused by previous overmetering is thus reduced, and a well-defined state of the catalytic converter is achieved.
According to the invention, after the conditioning step an adaptation mode is provided as a further method step. In the adaptation mode, a third target metering quantity and a third limit value are provided for a predefinable second time period. If a predefinable deviation of the efficiency value from the third limit value is identified after the second time period has elapsed, a deficient quality of the reducing agent solution is diagnosed. Due to the method steps preceding the adaptation mode, it is possible to rule out with a high level of certainty other sources of error as the cause of the originally detected malfunction, and based on the deviation of the efficiency value from the third limit value, a very reliable conclusion may be drawn concerning a deficient and in particular decreased quality of the reducing agent.
In one embodiment of the invention, a control unit having a computation model, which at least partially describes the SCR exhaust emission control system, is provided so that a target metering quantity is determinable, by means of which predefinable values or value ranges of a quantity of NH3 stored in the SCR catalytic converter, or an efficiency of a NOx conversion by the SCR catalytic converter, may be set in a regulated or controlled manner, and in addition limit values for the efficiency value of a properly operating SCR exhaust emission control system may be established. In particular for a normal metering mode, an adjustment is preferably made of the value of the NH3 filling level in the SCR catalytic converter, which is modeled by the computation model. An NH3 feed rate that results from the set metering quantity and a modeled NH3 consumption rate due to the reaction with NOx and oxygen and/or slip are preferably balanced by continuous integration, and the metering quantity is set in such a way that that an intended NH3 filling level in the catalytic converter, and thus an intended catalytic converter efficiency, are at least approximately achieved. In particular, in the event of decreased NH3 storage capacity of the SCR catalytic converter due to high catalytic converter temperatures, a switch to model-based efficiency control for achieving a catalytic converter target efficiency that is predefinable by the computation model may be provided. In any event, it is possible to determine or establish limit values for the efficiency value of a properly operating SCR exhaust emission control system by a linkage, carried out by the computation model, of measured values of instantaneous operating variables such as temperature, exhaust gas throughput, etc., and stored characteristic values in particular of the SCR catalytic converter. This achieves reliable error recognition and improved analysis of a detected error.
In another embodiment of the method, in the diagnostic mode a second target metering quantity is set, which is greater than the first target metering quantity by a factor of 1.1 to 5. This allows a brief first time period, and thus, despite the overmetering, also a low emission value of NH3 slip that may occur. Due to the NH3 overmetering in the diagnostic mode, the signal of the NOx sensor may be unambiguously interpreted despite the cross-sensitivity of the NOx sensor situated downstream from the SCR catalytic converter.
In another embodiment of the method, the first time period is selected as a function of the second target metering quantity in such a way that a quantity of NH3 metered with the reducing agent solution over the first time period corresponds to a quantity of NH3 that is storable in an SCR catalytic converter under the prevailing conditions with an NH3 storage capacity that is decreased to a tolerance limit. Accordingly, for establishing the first time period, a marginally aged SCR catalytic converter having an impaired NH3 storage capacity due to aging is assumed. For an impermissibly excessively aged SCR catalytic converter, the quantity of NH3 supplied with the reducing agent in the first time period would therefore result, at best, in noticeable or detectable NH3 slip. Due to establishing the first time period according to the invention, undesirable additional pollutant emissions in the diagnostic mode are avoided in a particularly reliable manner and catalyst aging is detectable.
In another embodiment of the method, for the conditioning, a target metering quantity that is less than the first target metering quantity or reduced to zero and/or an uncontrolled NOx emission, which is increased compared to normal internal combustion engine operation, is/are set. This allows particularly rapid reduction of a quantity of stored NH3 in the SCR catalytic converter which is increased due to the previous NH3 overmetering. On the other hand, a defined loading state for the SCR catalytic converter is achieved after conclusion of the third method step.
A further improvement in the reproducibility with regard to the provided conditioning of the SCR catalytic converter may be achieved when, in another embodiment of the method, the conditions for the conditioning are maintained until a quantity of NH3 stored in the SCR catalytic converter has fallen below a threshold value that is crucial for NH3 slip due to a reaction of NOx contained in the SCR catalytic converter with exhaust gas in the internal combustion engine.
In another embodiment of the method, an adaptable correction value that corrects the metering quantity is provided for the controlled or regulated setting of the quantity of NH3 stored in the SCR catalytic converter or of the efficiency of the SCR catalytic converter as a result of the metering quantity determined by the computation model. Thus, according to the invention the option is provided of making a correction that acts directly on the modeled metering quantity. Interfering influences, for example gradually progressive aging or drift effects, may thus be compensated for. In particular, compensation for interfering influences that may not be taken into account or that may be only insufficiently taken into account by the computation model is made possible. In this regard, it is particularly advantageous when, in another embodiment of the method, in the adaptation mode a controlled setting of the metering quantity for a predefinable target efficiency of the SCR catalytic converter and a determination of the actual efficiency of the SCR catalytic converter are carried out, and in addition a value of the correction value is determined that is or would be necessary for a predefinable approximation of the actual efficiency to the target efficiency, and subsequently a deficient quality of the reducing agent solution is diagnosed when the determined correction value is outside a predefinable target range. In this case, adaptation of the computation model by changing the correction value is not possible, or is possible only under unacceptable limitations. In particular, for a catalytic converter activity that is originally impermissibly decreased, an appropriately large, necessary change in the correction value in conjunction with the other information determined in the method according to the invention may be declared plausible only due to an impermissibly decreased quality in the reducing agent.
Advantageous embodiments of the invention are illustrated in the drawings and described below. The features stated above and to be explained below may be used not only in the particular stated feature combination, but also in other combinations or alone without departing from the scope of the present invention.